Radiation curing technology has been used in the graphic arts industry for more than 20 years. One key driver in the adoption of this technology for environmental and safety reasons is the reduction of volatile organic compounds, resulting from the elimination of solvents. The UV-technology has been limited to specific inkjet printing methods, which methods require either high printing temperatures or very low viscosity at room temperature. These methods can be used to form articles, such as an RP-product, RM-product or LMT-product. The UV-inks used there apply a solvent carrier. These solvents are used to control the ink viscosity and to promote adhesion to a substrate. It is further observed that typically curable compositions should contain as less solvent as possible, in order to obtain favourable physical and chemical properties of the cured composition. In addition the curable compositions should preferably contain a minimum amount of salt or no salt at all, since salt is regarded to be detrimental for favourable physical and chemical properties of the cured composition.
Inkjet printers can be divided into drop-on-demand (DOD) and continuous inkjet (CIJ) systems. In DOD systems, each drop is generated on demand as soon as the nozzle is positioned above the substrate. On the other hand, in CIJ systems a flow of ink drops is generated under constant and high pressure. The ink drops are charged and can selectively be deflected in order to obtain a desired droplet distribution across the substrate. Deflected drops can subsequently be recycled. To be charged, the ink needs to be electrically conductive.
As indicated above when used in a drop-on-demand (DOD) inkjet or in continuous inkjet (CIJ) printer, a (UV-curable) composition must have a sufficiently low viscosity. For most ink compositions, the applied viscosity is around 100 mPas at room temperature, though a more typically applied viscosity at printing temperature is less than 70 mPas in case of CIJ technology and less than 20 mPas in case of DOD technology. It will be appreciated that the viscosity of a composition can be influenced by the inkjet printing temperature.
Most conventional film forming compositions for other applications such as coatings, however, have a significantly higher viscosity, which make them unsuitable for inkjet printing processes.
Oxygen inhibition is another factor that impacts the performance of UV-curable materials. Rapid cure speeds are required to insure good print quality and production efficiency. As a result, highly functional materials are necessary. However, these materials must not exhibit high film shrinkage, which would negatively impact the ink and the adhesion of the ink. Therefore these materials are preferably applied at or close to the curing temperature. In this respect DOD technology is less favourable, as this technology needs to use the unfavourable high printing temperatures. Alternatively DOD technology needs to use very low viscosity formulations at room temperature leading to very dense networks giving brittle materials and hence are unsuitable for 3D modelling.
In conventional UV inks typically a modified epoxy, a urethane, or a polyester acrylate is used as the backbone of the ink composition. Specific epoxy chemistry, however, suffers from high molecular weights and viscosities, high film shrinkage and limited pigment capability. Hence, epoxy-based compositions are not suited for inkjet applications, since for such applications they have to contain high amounts of solvents. Furthermore, for typical cationic ring opening polymerization (epoxides and oxetanes), shrinkage may be very low, that is much lower than for acrylates and may therefore be very interesting for 3D, RP and RM. Drawbacks of these systems are, however, that polymerisation in these latter cases is relatively slow and inhibited by proton acceptors (such as water).
The use of conventional urethane and polyester acrylates brings about better overall physical properties, but their viscosities are still very high, which makes them not particularly suited for inkjet applications.
Furthermore, it is observed that conventional reactive monomers and oligomers that are typically used in UV-systems are not water-soluble, whereas traditional photo initiators have a poor compatibility with water, since they do not disperse well in water, e.g. because of their polar nature.
In an article by J. Klang and J. Balcerski from Sartomer, Pa., USA (2004), UV-curable compositions are described, which contain a monomer, an oligomer, photo-initiators, pigments and additives. These compositions are preferably free of solvent to reduce or minimize the amount of volatile organic compounds that are released upon curing the article obtained. In addition, these compositions have a low viscosity to allow their application in inkjet systems. The solvent free compositions can in particular cases be used for DOD-systems, though still relatively high application temperatures are required to decrease the viscosity of the composition.
It is observed, however, that such solvent free compositions cannot be used in continuous inkjet printing, as these compositions are not sufficiently conductive or not conductive at all.
A further disadvantage is that these compositions, when used in drop-on-demand inkjet systems, need to be heated to about 80° C. in order to give these compositions the required viscosity. An elevated temperature is in general however malicious for such compositions, because not only the stability, but also the homogeneity of such compositions at such temperatures deteriorates.
In addition, the processing of compositions at higher temperature causes the compositions to thermally shrink when deposited, which is unfavourable and therefore unwanted. Moreover, in order to heat the composition much energy is consumed, which is a further disadvantage.
U.S. Pat. No. 4,990,360 discloses electrically conductive compositions containing acrylate functional organosiloxane/oxyalkylene copolymers and a solubilized lithium salt. However, the use of these compositions is limited to the formation of gels upon curing. It is therefore not possible to form solid articles with these compositions. Furthermore, the compositions mentioned herein are solvent-based, which is not desirable, e.g. from an environmental point of view. Another disadvantage is that these compositions have only a limited range of viscosities. Furthermore the materials described in this patent are polymeric and would have viscosities that are far too high for solventless inkjet printing.